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Full text of "The Cambridge Natural History"

HOOFS,   NAILS   AND   CJLAWS

important part in their life, and it is perhaps worthy of note
that birds with highly-variegated plumage are provided only
with the uropygial gland, while mammals with usually dull and
similar coloration have a great variety of skin glands. Scent
is no doubt a sense of higher importance in mammals than in
birds. The subject is one which will bear further study.
Mails and Claws.—Except for the Cetacea (where rudi-
ments have been found in the foetus), the extremities of the
fingers and of the toes of mammals are covered by, or encased in,
horny epidermic plates, known as nails, claws, and hoofs.
The variety in the shape and development of these corneous
sheaths to the digits is highly characteristic of mammals as
opposed to lower Vertebrates. If we take extreme cases, such as
the nail of the thumb in Man, the hoof of a Horse, and the claw of
a Cat, it is easy to distinguish the three kinds of phalangeal horny
coverings. But the differences become extinguished as we pass
from these to related types. The nail of the little finger in Man
approaches the claw-like form; and the hoofs of the Lama arc
almost claws in the sharpness of their extremities. On the
whole it may be said that claws and hoofs embrace the bono
which they cover, while nails lie only upon its dorsal surface.
The form, of the distal phalanx which bears the nail shows,
however, two kinds of modification which do not support such a
classification. When those phalanges are clad with hoofs or
covered by a nail they end in a rounded and flattened termina-
tion. On the other hand, when they bear a claw they are them-
selves sharpened at the extremity and often grooved above.
The Marsupium.—It rnay appear to be unnecessary at this
juncture to speak of the marsupial pouch, which is so usually
believed to be a characteristic of the group Marsupialia. Hudi-
ments of this structure have, however,, been recently discovered
in the higher mammals, and, as Br. Klaatsch l has remarked, all
researches into the " history of the mammals culminate in the
question whether the placental mammals pass through a mar-
supial stage or not." We cannot, therefore, look upon the
marsupial pouch as a matter affecting only the Marsupials,
though it is true that this organ is at pre$e;nt functional only in
them and in the Monotreniata.
1 "Uber Marsirpialrudlmeate bei
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